Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations each radiating to provide coverage in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In practice, a base station may be configured to provide service on multiple carrier frequencies or “carriers.” Each carrier could be a time division duplex (TDD) carrier that defines a single frequency channel multiplexed over time between downlink and uplink use, or a frequency division duplex (FDD) carrier that defines two separate frequency channels, one for downlink communication and one for uplink communication. Each frequency channel of a carrier may then occupy a particular frequency bandwidth defining a range of frequency at a particular position (e.g., defined by a center frequency) in the radio frequency spectrum. As such, carriers may differ in width from each other and reside at different frequencies than each other. For instance, industry standards define certain TDD carriers to be 20 MHz wide and to reside in the relatively high 2.5 GHz frequency band, and certain FDD carriers to be 5 or 10 MHz wide (per channel) and to reside in the relatively low 800 MHz frequency band. Other examples exist as well.
Each carrier may also define various logical channels to facilitate communication between the base station and one or more served UEs. For instance, on the downlink, a carrier may define a reference channel on which the base station broadcasts a reference signal useable by UEs to detect and evaluate coverage, various other downlink control channels to carry control signaling (such as resource-scheduling directives) to UEs, and one or more shared or traffic channels for carrying bearer data (e.g., user or application level data) to UEs. And on the uplink, a carrier may define one or more uplink control channels to carry control signaling (such as resource scheduling requests, channel state reports, and the like) from UEs, and one or more shared or traffic channels for carrying bearer data from UEs.
When a UE enters into coverage of a base station on a particular carrier, the UE may attach or register with the base station on that carrier, and the base station may then serve the UE on that carrier. Further, under certain air interface protocols, a base station may be able to serve a UE concurrently on multiple carriers, to help increase the effective bandwidth and associated throughput available to the UE. For instance, if a UE is attached with a base station on a first carrier, the base station may then add a second carrier to its service of the UE so as to then provide the UE with “carrier aggregation” service on a combination of the first carrier and the second carrier. In that arrangement, the first carrier may be considered the UE's primary carrier or primary cell (PCell), and the second carrier may be considered the UE's secondary carrier or secondary cell (SCell). Depending on the carrier aggregation implementation, the SCell might be used principally for downlink communication (to increase the UE's downlink throughput) rather than for uplink communication, and the PCell may carry some or all control signaling related to the SCell (in addition to control signaling related to the PCell).